Automatic testing of redundant switching element and automatic switchover

ABSTRACT

A non-disruptive, on-line testing and switchover method and apparatus in a high availability, fibre channel switching environment. In a network having an active switching element, a redundant switching element and a port, one aspect of the present invention provides for verifying a working data path from the port to the redundant switching element, and thereafter verifying a working control path to the redundant switching element. Both verification tests take place without interrupting operation on the network. In addition, another aspect of the present invention provides for recognizing a failure occurrence in the active switching element, and thereafter switching to the redundant switching element with the potential for minimal frame loss.

RELATED APPLICATION

[0001] The present application is a divisional application of co-pendingU.S. patent application Ser. No. 09/829,448, filed Apr. 9, 2001, whichis incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates, in general, to the field of fibrechannel switching technology. More particularly, the present inventionrelates to non-disruptive, on-line testing and switchover in a highavailability, fibre channel switching environment.

[0003] Fibre Channel is a high performance, serial interconnect standarddesigned for bi-directional, point-to-point communications betweenservers, storage systems, workstations, switches, and hubs. It offers avariety of benefits over other link-level protocols, includingefficiency and high performance, scalability, simplicity, ease of useand installation, and support for popular high level protocols.

[0004] Fibre channel employs a topology known as a “fabric” to establishconnections between ports. A fabric is a network of switches forinterconnecting a plurality of devices without restriction as to themanner in which the switch can be arranged. A fabric can include amixture of point-to-point and arbitrated loop topologies.

[0005] In Fibre Channel, a channel is established between two nodeswhere the channel's primary task is to transport data from one point toanother at high speed with low latency. The Fibre channel switchprovides flexible circuit/packet switched topology by establishingmultiple simultaneous point-to-point connections. Because theseconnections are managed by the switches or “fabric elements” rather thanthe connected end devices or “nodes”, fabric traffic management isgreatly simplified from the perspective of the device.

[0006] In a high availability, fibre channel switching environment, asecond set of “redundant” elements are provided in the event of afailure condition. The number and make-up of the redundant elementsparallel the primary elements, and operate as back-up resources if theprimary elements fail. As such, in the event of such a fail condition, aswitchover to the redundant elements can greatly minimize the loss oftransmitted data frames.

[0007] In prior approaches, the redundant elements in a highavailability environment are passive in nature. This approach provides asecond set of elements that remain inactive until the occurrence of afail condition. In such an all-or-nothing passive environment, itbecomes necessary to take a system offline to perform the necessarytests on the elements to determine if they are still in workingcondition. Since taking a system offline is often not a viable option,the unfortunate effect is a lack of a significant method of testing theredundant elements. As a result, switching over to the redundantelements may result in a situation where an element is either partiallyor completely non-functional.

[0008] Another limitation of prior systems is that during switchover tothe redundant elements in a failure condition, frames were lost, sincethere was no implementation of a completely seamless method oftransition to the redundant elements. It is well understood that for acatastrophic failure event, immediate switchover is of paramountimportance. However, for a non-catastrophic failure, the significance ofimmediacy becomes secondary to the importance of maintaining theintegrity of a frame transmission.

SUMMARY OF THE INVENTION

[0009] The on-line testing and switchover design of the presentinvention provides a solution to the aforementioned problems which isvastly superior to anything currently available. It not only solves thecritical situation where, after switchover, an element is eitherpartially or completely non-functional, but it does so in an extremelyefficient manner without requiring any significant design changes andwith only a relatively straightforward alteration to existing processesfor networking in a high availability, fibre channel switchingenvironment.

[0010] The present invention advantageously provides a proactivemanagement approach for verifying the integrity of a redundant switchingelement prior to actual use. More particularly, the on-line testing andswitchover design of the present invention offers independent,non-disruptive online testing via an independent request/responsesignaling interface on a per port basis. Since the invention operates ona per port basis, it is unnecessary to shut down the other operatingports to carry out the testing operation, and therefore the ports cancontinue to transmit data. In addition, the invention advantageouslyprovides the ability to switch over to a redundant switching element ona frame boundary, thereby reducing the possibility of losing a frameduring the transition from a currently functioning path to a backuppath.

[0011] Particularly disclosed herein is a method of performing anonline, non-disruptive health check test in a fully redundant fibrechannel switching network having an active switching element and aredundant switching element. The health check test is performed on theredundant switching element without the need to take the networkoffline. After a first and second status condition are met prior to theexpiration of a timer, a working data path from a port to the redundantswitching element is verified and the condition is recorded. Then, aworking control path to the redundant switching element is verified andthe condition is recorded.

[0012] In another aspect, the present invention provides a method ofperforming automatic switchover in a fully redundant fibre channelswitching network having an active switching element, a redundantswitching element and a port having switching element logic embodiedtherein. To begin, a failure condition is detected in the activeswitching element. The failure condition is transmitted to the portlogic. In response to a possible problem with the active switchingelement, software notifies the port logic to perform the switchover.Notification may originate from either the active switching element orport logic via software collection. In response, the port is queried todetermine if the port is transmitting a frame. If a port is in theprocess of transmitting a frame, then the automatic switchover isdelayed until the port is not transmitting a frame.

[0013] Still further disclosed herein is a fibre channel switchingnetwork having increased bandwidth capacity. The network comprises afirst switching element, a second switching element, a first readercoupled to the first switching element and a second reader coupled tothe second switching element. Continuing, the switching network also hasa buffer memory for storing frames, wherein the buffer memory is coupledto the first reader and the second reader. The network also has a writercoupled to the buffer memory, wherein the writer directs a buffercontrol to store frames in the buffer memory. The first reader or thesecond reader directs the buffer control to retrieve a frame from buffermemory and pass the frame to the respective reader for transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The aforementioned and other features and objects of the presentinvention and the manner of attaining them will become more apparent andthe invention itself will be best understood by reference to thefollowing description of a preferred embodiment taken in conjunctionwith the accompanying drawings, wherein:

[0015]FIG. 1 is a block diagram of two switching elements, a singlefibre port module having switching element control logic, an RX port anda TX port;

[0016]FIG. 2 is a block diagram showing the functionality of a fibreport module;

[0017]FIG. 3 is a flow chart for the health check test;

[0018]FIG. 4 is a flow chart for the health check test;

[0019]FIG. 5 is a prior art block drawing of a current configuration forswitching elements;

[0020]FIG. 6 is a block diagram of a multi-drain configuration in whichboth switching elements are active thereby increasing bandwidth; and

[0021]FIG. 7 is a flow chart for the automatic switchover.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0022]FIG. 1 shows a fully redundant, generalized fibre channelswitching environment implementing the method and systems of the presentinvention. FIG. 1 illustrates a number of devices and connectionsbetween the devices that are indicated by connecting lines. Inparticular, two switching elements or SBAR's, SBAR0 25 and SBAR1 30, areshown coupled to a fibre port module 10 over receive paths or inboundpaths 35, 40 and transmit or outbound paths 50, 55. Fibre port module 10has port logic embodied thereon, however, only the SBAR controlinterface logic of fibre port module 10 is shown in FIG. 1. In addition,fibre port module 10 comprises receive (RX) block 20 and transmit (TX)block 15 for coupling fibre port module 10 to and from the backplane.

[0023] SBAR0 25 and SBAR1 30 each have a fully independentrequest/response interface mechanism for processing requests andresponses to and from fibre port module cards embodied within fibre portmodule 10. As shown in FIG. 1, SBAR0 25 communicates with fibre portmodule 10 over request line 60 and response line 65. Similarly, SBAR1communicates with fibre port module 10 over request line 75 and responseline 70. For example, fibre port module 10 may transmit a request forconnection to SBAR1 30 over request line 75. SBAR1 30 responds overresponse line 70 with a response, such as “destination port busy.” Inone embodiment, data on both request lines 60, 75 and response lines 65,70 is serially encoded for transmission between fibre port module 10 andSBAR0 25 and SBAR1 30.

[0024] Also shown in FIG. 1 is data control multiplexer 45, which iscoupled to fibre port module 10 by connection 80. Data controlmultiplexer 45 is also coupled to SBAR0 25 via outbound path 50, and toSBAR1 30 via outbound path 55. Connection 80 is part of outbound paths50, 55 respectively depending upon which SBAR is active. Using logicthat is not shown, data control multiplexer 45 transmits data from theactive SBAR, either SBAR0 25 or SBAR1 30, to fibre port module 10. Thelogic stores the identity of the active SBAR, which provides datacontrol multiplexer 45 with sufficient information so as to pass throughthe data from either outbound path 50 or outbound path 55 to fibre portmodule 10.

[0025] Continuing with FIG. 1, one example of a frame transmission froma port to a redundant network of switching elements will now beexplained in detail. In operation, a single frame appears at fibre portmodule 10 intended for a particular destination port. Fibre port module10 sends a connection request to the active switching element. Logicwithin fibre port module 10 indicates which SBAR is active and whichSBAR is redundant. In this example, SBAR1 30 is the active switchingelement and SBAR0 25 is the redundant switching element. For purposes ofbeing complete, since SBAR0 25 is a redundant switching element, thedefault condition is that for each port, the redundant switching elementis looped back to itself, i.e. RX 20 is looped back to TX 15 for SBAR025.

[0026] Continuing with the example, since SBAR1 30 is the activeswitching element, fibre port module 10 sends a connection request,sb_sbar1_req, to SBAR1 30 over request line 75 seeking to transmit thesingle frame to the particular destination port. SBAR1 30 investigatesand determines if the particular destination port is available. If theparticular destination port is unavailable, SBAR1 30 responds with a“destination port busy” response, and a connection is not established.If the particular destination port is available, an appropriate responseis sent to fibre port module 10 over response line 70 indicating thatthe connection is established. The fibre port module then prepares tosend the frame to the switching elements. In this case, the switchingelements are now deemed busy and frames cannot be transmitted throughthe busy port until the connection is terminated.

[0027] The single frame is then transmitted from RX port 20 over inboundpaths 35, 40. Since the illustrated example is a fully redundant fibrechannel switching network, the frame is transmitted over both inboundpath 40 to active switching element SBAR1 30 as well as inbound path 35to redundant switching element

[0028] SBAR0 25

[0029] The return paths from SBAR0 25 and SBAR1 are outbound path 50 andoutbound path 55 respectively through data control multiplexer 45 overconnection 80 to TX port 15. Since redundant switching element 25 islooped back to itself for this port, the single frame is transmittedthrough SBAR0 25 and appears at data control multiplexer 45. As statedpreviously, data control multiplexer 45 contains logic to ascertain theidentity of the active switching element, in this case SBAR1 30. SinceSBAR0 25 is the redundant switching element, the frame from SBAR0 25 isdiscarded, and the frame transmitted from SBAR1 30 is passed through toTX port 15 of fibre port module 10.

[0030]FIG. 2 illustrates another aspect of fibre port module 10 forimplementing the method and systems of the present invention. Fibrechannel front end (FE) 200 provides an interface to a central processor(CTP). The main element of the CTP is the system services processor(SSP) that provides processing power. In the case of processing frametraffic, FE 200 provides independent, symmetrical RX and TX interfacesto carry frame data. Coupled to FE 200 is frame writer 205. Inoperation, FE 200 forwards a frame to frame writer 205 for storage inbuffer memory 210. Frame writer 205 stores the frame in the nextavailable buffer in buffer memory 210, modifies a free buffer listaccordingly and passes the buffer location to queue manager (Q_(m)) 220.Frame reader 215 is responsible for reading frames stored in buffermemory 210 and transferring them to SBAR 250.

[0031] Continuing with FIG. 2, Q_(m) 220 is the central core of fibreport module 10. Q_(m) 220 is responsible for building queues of receivedframes for destination ports from the information received from framewriter 205 as well as providing buffer location information to framereader 215. Q_(m) 220 also interfaces with switching element (SBAR) 250through SB control logic 225 for the establishment of connections as asource.

[0032] Transmit handler 230 provides the interface for the transmissionof the frames at the port. The transmit handler logic interfaces to bothSBAR's in a fully redundant switching network. However, through CTPcontrol, only one interface is active at a time.

[0033] Health Check Test

[0034] In one embodiment, a health check test is initiated by a CentralProcessor (CTP) via a control register bit in the SB logic containedwithin fibre port module 10. The initiation of the health check test maybe software driven by operational or online software, and as such, thetest could be activated on a regular schedule, e.g. hourly, weekly, ormonthly to name a few. By activating the control register bit in the SBlogic of fibre port module 10, the CTP is in effect “taking control” ofthe port and the associated paths for a brief period of time to performthe health check test. During this brief period of time the port isunder control, the port remains “offline” to new requests. The term“offline” simply means that the port remains unavailable for sendingframes to the active switching element. However, the port still receivesand buffers new frames via the front end logic until the port runs outof available buffers. The offline condition occurs only when the firststatus condition described later is satisfied.

[0035]FIGS. 3 and 4 illustrate, using flow charts, one embodiment of ahealth check test in accordance with one aspect of the presentinvention. For purposes of clarity, the health check test is bestexplained in two parts. The first part of the test verifies theintegrity of a data path between a port and a redundant switchingelement. The second part of the test verifies the integrity of a controlpath between a port and a redundant switching element. In combination,the two parts of the illustrated health check test evaluate theoperating condition of a redundant switching element, therebydetermining if it would function properly in a switchover condition. Thetwo parts are explained in greater detail in the sections that follow.

[0036] Data Path Test

[0037] Once the fibre port module 10 acknowledges that the CTP has set aregister bit in the SB logic to begin the health check test, the logicbegins the process of taking control of the port and the correspondingdata and control paths to both the active and redundant switchingelements. In accordance with one important aspect of the invention,certain limits are in place as to how long the port is allowed to remainin test mode since the test is designed to minimize disruption to theport. In most cases, the limits are controlled by timers that, ifexpire, operate to return the port to active status and terminate thehealth check test.

[0038] As shown in the flow chart shown in FIG. 3, the fibre port module10 determines, through its internal port logic, whether the CTP hasinitiated a health check test (step 305). If a health check test hasbeen initiated, the SB logic within fibre port module begins the processof taking control of the port and associated paths. The control logic,and more particularly a state machine within the control logic, monitorsthe status of the inbound and outbound paths (step 310) for theoccurrence of a frame.

[0039] A first timer, or inactivity timer, starts a terminationcountdown (step 310) at the same time the control logic beginsmonitoring both the status of the inbound or transmit (TX) path and thestatus of the outbound or receive (RX) path. The inactivity timer placesa limit on the amount of time the port will monitor the inbound andoutbound path for a frame transmission before timing out and returningthe port to normal operation. In one embodiment, the timer is set toexpire after 100 microseconds. Advantageously, the inactivity timerprevents stalling the port while waiting for the correct transmit andreceive conditions to be met on the inbound and outbound paths.

[0040] Continuing with the example of FIG. 3, the port logic of fibreport module 10 monitors the outbound path for the occurrence of a frame(step 315). It should be understood that since the logic is monitoringboth paths simultaneously, the order of the steps of monitoring theinbound and outbound paths is arbitrary and is illustrated in FIG. 3 inany particular order solely for purposes of explanation. If a frame isdetected on the outbound path, the logic continues to monitor theinbound path for the occurrence of a frame (step 330).

[0041] If a frame is then detected on the inbound path, the logic hasdetected the correct status of both the RX and TX paths. The logic thenperforms several operations to control the port and paths (step 340)that are explained in the paragraphs that follow. If the first timerexpires at any point prior to detecting of a frame on both the inboundand outbound paths (steps 325, 345 and 350), the port immediatelyreturns to normal operations and the health check test is terminated(step 360).

[0042] Referring back to the flow chart of FIG. 3, a first statuscondition, as that term is used herein, describes the occurrence of aframe on the RX path prior to the expiration of the inactivity timer(steps 320 and 330). The first status condition triggers the SB logic ofthe port to block any new requests from being sent through the port. Ifany new requests are received, the new requests will be placed in queueuntil the completion of the health check test. The new requests are thesent when the port resumes normal operation, which is either completionor termination of the health check test.

[0043] The occurrence of a first status condition also provides for adelay of a certain length of time to allow the completion of any frametransmission on the RX path. In one embodiment, the delay is for aperiod of 20 microseconds. In any case, the delay should be at leastlong enough to allow for the maximum length of a fibre channel frame orthe length of time necessary to make certain the RX path is clear.

[0044] Referring again to FIG. 3, a second status condition, as thatterm is used herein, describes the appearance of a frame on the TX pathprior to the expiration of the inactivity timer (steps 315 and 335). Thesecond status condition triggers the SB logic of the port to hold offclearing the ports busy bit. The term is busy bit is used as anindication to the switching element that the port is either available orunavailable to transmit a frame. Once the health check test is eithercompeted or terminated, the ports busy bit is cleared allowing the portto resume normal operations.

[0045] Once the first status condition and second status condition aresatisfied, the port logic is temporarily in control of fibre port 10 andthe associated data paths and control paths. The health check test maybe engaged by the port logic (step 355). In one embodiment, prior toengaging the health check test, the active data path is changed from theactive switching element to the redundant switching element. In theillustrated example of FIG. 1, a data path multiplexer 45 switches tothe opposite of the programmed register value, thereby setting theinbound data to the port to come from the redundant switching element.

[0046] The port logic instructs the RX logic to transmit a test datapattern on the RX path to the redundant switching element. Concurrently,the port logic instructs the TX logic to monitor the TX path for theappearance of the test data pattern. In one embodiment, the test datapattern is programmed into a shared register in the port. Since thereceive and transmit ports are looped together for a redundant switchingelement, the data sent on the RX side will appear at the TX side if theswitching element is in operating condition.

[0047] A second timer, or an expect frame time out timer, starts at thesame time the port logic instructs the RX logic to transmit the testdata pattern. The expect frame time out timer places a limit on theamount of time the port logic will monitor the inbound path for the testdata pattern before timing out and proceeding to the control path test.In one embodiment, the timer is set to expire after 10 microseconds.

[0048] Referring now to the flow chart of FIG. 4, the SB logic in theport monitors the receipt or non-receipt of the test data pattern. Ifthe test data pattern is detected at the port prior to the timeout ofthe second timer (step 400), a data path good condition of the redundantswitching element is logged at the port. However, if the second timerexpires prior to the detection of the test data pattern (step 405), adata path bad condition is logged at the port. In either case, once thecondition of the data path is recorded, the data path test is completeand the control path test begins.

[0049] Control Path Test

[0050] Continuing with FIG. 4, the illustrated control path test is arelatively quick method for determining the integrity of the controlpath between a port and a redundant switching element. Since the test isa continuation of the data path test, it should be understood that eventhough the data path test is complete, the port logic is still incontrol of fibre port 10 and the associated data paths and controlpaths.

[0051] As shown in FIG. 4, a connection request is sent from the port tothe redundant switching element (steps 410 and 415). Since the defaultcondition for a redundant switching element is to set all the busy bitsto an on position, a connection request from the port should elicit a“destination port busy” response.

[0052] A third timer, or an expect response time out timer, starts atthe same time the connection request is sent from the port to theredundant switching element. The expect response time out timer places alimit on the amount of time the port logic will monitor the control pathfor the “destination port busy” response before timing out and loggingthe results of the test. In one embodiment, the timer is set to expireafter 3 microseconds.

[0053] Referring now to the flow chart of FIG. 4, the SB logic in theport monitors the receipt or non-receipt of the response to theconnection request. If the “destination port busy” response is detectedat the port prior to the timeout of the third timer (steps 420 or 435),a control path good condition of the redundant switching element islogged at the port. However, if the third timer expires prior to thedetection of the “destination port busy” response or if some othercorrupted or incorrect response is received (steps 425 and 440), acontrol path bad condition is logged at the port. Upon recording thecondition of the control path, the control path test is complete. Atthis point, the health check test is over and the port resumes normaloperation.

[0054] Multi-Drain Concept

[0055]FIG. 5 is a block diagram of prior art fibre channel switchingsystem. Buffer memory 505 is coupled to a single frame writer 500 and asingle frame reader 510. SBAR0 515 and SBAR1 520 are both coupled toframe reader 510. In operation, frames are stored in buffer memory 505until they are switched through either SBAR0 or SBAR1. However, sinceonly a single frame reader 510 is coupled to both SBAR0 515 and SBAR1520, only a single switching element is operative at a given moment.Hence, only one read and one write operation is allowed in aparticularly defined micro-cycle.

[0056]FIG. 6 is a block diagram of a fibre channel switching systemaccording to one embodiment of the present invention. Instead of asingle frame reader coupled to the switching elements, each switchingelement has a frame reader operatively coupled thereto. As illustratedin FIG. 5, SBAR0 625 is coupled to frame reader 610 and SBAR1 620 iscoupled to frame reader 615. In operation, a frame can be transmittedfrom buffer memory 605 through frame reader 610 to SBAR0 625. At thesame time, a frame can be transmitted from buffer memory 605 throughframe reader 615 to SBAR1 620. This type of operation allows forincreased bandwidth since two switching elements are operating at thesame time. In addition, in a failure situation involving one of theswitching elements, a second switching element remains operatingresulting in decreased bandwidth but no down time.

[0057] Automatic Switchover

[0058] Referring now to FIGS. 1 and 7, one embodiment of the automaticswitchover aspect of the present invention is described. The automaticswitchover aspect switches from an active switching element, such asSBAR1 30, to a redundant switching element, such as SBAR0 25, inresponse to a failure condition. The switchover is carried out so thatany frames that may be transmitting at the time of the failure arecompleted. For example, only one port might be bad, but the other portsare still reliably transmitting frames. If the failure is not timecritical requiring immediate switchover, the present invention allowsfor the continued operation of the switching element until all ports areout of frame.

[0059] To begin, a failure condition is detected at a switching element(step 700). Several examples of a non-catastrophic failure conditioninclude a bad port, one or more ports identifying a channel from theswitching element as bad while other channels continue to operate orfailure to clear the busy bit for one or more ports to name a few. Ifthe failure is a catastrophic failure requiring immediate attentionwithout regard for the status of the frame transmission (step 705), suchas loss of power, all busy bits not working or component failure thatkeeps an active switching element from performing an operational task toname a few, the switchover is immediate. However, if it is determinedthat the switchover can be delayed for a period of time without harmingthe integrity of the switching system, the automatic switchover featureprovides a method of waiting until all frames are transmitted beforeperforming the switchover.

[0060] Continuing with FIG. 1, SB logic within fibre port module 10receives a signal that indicates the identity of the active SBAR. In theillustrated example, SBAR1 30 is the active SBAR. When a changeover isto occur, the signal will indicate the redundant SBAR is the new activeSBAR. In the illustrated example, SBAR0 25 is the new active SBAR. TheSB logic queries the TX port to determine if a frame is currently beingtransmitted over outbound path 80. If the port is “in frame” orcurrently transmitting a frame, then the SB logic will wait a period oftime before executing the switch until a busy bit is received from theTX port. In one embodiment, the period of time is 10 microseconds.

[0061] Once the frame transmission is complete and the busy bit isreceived and detected by the SB logic, the SB logic proceeds to changethe inbound data select to the value indicated by the main control bit.The inbound data select identifies the active SBAR, in this case theformer redundant switching element SBAR0 25. The switchover has takenplace and the busy bit is cleared to indicate the port is available totransmit frames again.

[0062] While there have been described above the principles of thepresent invention in conjunction with a specific embodiment, it is to beclearly understood that the foregoing description is made only by way ofexample and not as a limitation to the scope of the invention.Particularly, it is recognized that the teachings of the foregoingdisclosure will suggest other modifications to those persons skilled inthe relevant art. Such modifications may involve other features whichare already known per se and which may be used instead of or in additionto features already described herein.

[0063] Although claims have been formulated in this application toparticular combinations of features, it should be understood that thescope of the disclosure herein also includes any novel feature or anynovel combination of features disclosed either explicitly or implicitlyor any generalization or modification thereof which would be apparent topersons skilled in the relevant art, whether or not such relates to thesame invention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as confronted by thepresent invention. The applicants hereby reserve the right to formulatenew claims to such features and/or combinations of such features duringthe prosecution of the present application or of any further applicationderived therefrom.

What is claimed is:
 1. A fibre channel switching network havingincreased bandwidth capacity comprising: a first switching element; asecond switching element; a first reader coupled to said first switchingelement; a second reader coupled to said second switching element; abuffer memory for storing frames, said buffer memory coupled to saidfirst reader and said second reader; and a writer coupled to said buffermemory, said writer for directing said buffer memory to transmit framesfrom said buffer memory to either said first reader or said secondreader.
 2. The fibre channel switching network of claim 1, furthercomprising; a third switching element and a third reader coupled to bothsaid third switching element and said buffer memory, wherein said writerdirects said buffer memory to transmit frames from said buffer memory tosaid third reader.